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Some OAuth2 providers return quite large structured tokens >32767 bytes.
Gitea currently has a fixed maximum of 32767 bytes for these and
unfortunately due to the convoluted nature of the dependent libraries the
error returned is rather opaque.

Here we manage the error a little better - detecting the rather opaque
github.com/gorilla/securecookie.errEncodedValueTooLong and converting
it to a more readable error.

Further we provide a configurable option to increase the maximum size of
the provided OAuth2 tokens.

Fix #9907
Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>